Double glazed windows have been in use for some time as described in "Windows -- Performance, Design and Installation" by Beckett and Godfrey, Van Nostrand Reinhold, New York (1974). A double glazed window consists of two parallel panes of glass which are spaced apart to leave an air space between the two panes and having the periphery of the space between the two panes closed by a moderately flexible sealant which extends between the two panes along their peripheries, holding them apart and enclosing a generally rectangular parallelepiped body of air between the two panes. Polybutene resins and polysulfide resins are commonly used as sealants in the construction of the double glazed windows.
The purpose of a double glazed window is to provide thermal insulation and insulation against noise. At the time of their writing, Beckett and Godfrey noted the problem of condensation of water vapor contained in the air in the space between the two panes when the temperature of the air space drops below its dew point and noted that, "In the context of windows, condensation can occur both on the surface of the glass and on the frame facing the room and with double windows, additionally within the cavity between the two glazings. Whenever it occurs, the results can be very troublesome, impairing the view out and leading to the deterioration of the paint work and window frames." They note also that dehydrating agents and desiccants such as silica gel may be placed in the cavity to adsorb moisture from the entrapped air and so contribute to the suppression of condensation.
Double glazed windows, commonly referred to as sealed insulating glass, commonly have a narrow body of solid adsorbent positioned in the space between the two panes and lying in close proximity to the sealing resin which both holds the two panes together and apart. The solid adsorbent is commonly held in a generally rectangular aluminum tube which is either perforated or not completely sealed so that the enclosed air may have contact with the adsorbent and this adsorbent may lie along all or part of the interior periphery of the sealed insulating glass.
Passage of time and acquisition of experience has shown that condensation of water vapor is not the sole condensation problem attending the use of double glazed windows but that additionally over a period of time some decomposition of organic sealants occurs releasing condensible vapors such as hydrocarbon vapors or organic sulfide vapors which may also condense on the interior surface of the glass panes. It is current practice to use as the adsorbent to suppress condensation, a synthetic zeolite, sometimes referred to as a molecular sieve, or silica gel, or activated alumina, or a mixture of synthetic zeolite and a second adsorbent such a silica gel. The use of a second adsorbent to supplement large pore molecular sieve adsorbents was based on the observation that the rapid adsorption of water vapor by the molecular sieve reduces its capacity for adsorption of hydrocarbon vapors or organic sulfides. The molecular sieve which have been employed have all had pore diameters of such size that nitrogen molecules and oxygen molecules as well as water vapor molecules were able to penetrate the pores of the adsorbent.
The use of molecular sieve zeolites of this character has given rise to a problem which appears not to have been recognized heretofore, but if it has been recognized, either it has been ignored or no solution for it has been proposed so far as is now known.
The relatively recent discovery of the "energy problem" pertends a great increase in the use of double glazed windows going far beyond current use in predominantly glass covered skyscrapers and extending to extensive use in dwelling houses and apartments.
The seemingly certain large increase in the use of double glazed windows suggests that they be constructed to provide maximum efficiency and life and suggests that the problem which attends the use of adsorbents which adsorb not only water vapor but also nitrogen and oxygen can no longer be ignored.
The problem may be defined as follows. In the northern part of the temperature zone the temperature of the air enclosed between the two panes of a double glazed window may easily rise to 110.degree. F. or above on a warm summer day and may fall to 0.degree. F. or below on a cold winter night. At the lower temperatures in this range, the molecular sieve zeolites currently used adsorb not only water vapor but also adsorb substantial amounts of oxygen and nitrogen. At higher temperatures adsorbed oxygen and nitrogen tend to be released from the adsorbent and migrate back into the gas space enclosed between the two panes. The resultant cycles of adsorption and desorption with temperature variation, both day-night variation and seasonal variation, results in significant changes in the pressure of the air enclosed between the two panes. The pressure of the enclosed air may commonly vary by 6% or more merely as a result of adsorption or desorption of oxygen and nitrogen. This pressure variation is, of course, amplified by the affect of temperature. For example, with rising temperature, not only are nitrogen and oxygen desorbed from the molecular sieve zeolites now in use, but in addition the rise in temperature itself causes an increase in the pressure of the gas enclosed between the two relatively rigid panes. Conversely, with falling temperature, the adsorption of nitrogen and oxygen increases with a resultant lowering of the pressure of the gas in the space enclosed between the two panes and in addition, the lowering of the temperature itself causes a further reduction in the pressure of the enclosed gas. These continuing fluctuations in pressure cause some distortion of view through the double glazed windows and, further, these fluctuations cause a backward and forward movement of the panes themselves with a resultant tendency to weaken the seals between the two panes formed by the resins and ultimately to permit openings between the exterior air and the enclosed air through the sealing resin which permits the enclosed space to more or less breathe with the result that over a period of time capacity of the adsorbent to take up additional water vapor introduced through such breathing is exhausted.